Liquid crystal display devices have generally been used for conventional low-profile and light-weight flat-panel displays. The liquid crystal display device has, however, a problem in that it has a smaller angle of field and poor response characteristics because it controls the transmitted light with the orientation direction of the liquid crystal. Active matrix organic EL display devices have recently attracted attention which have a large angle of field and high response characteristics. The organic EL element is a self-light-emitting element based on the principle in which the electric field applied can bring the holes injected from the anode and the electrons injected from the cathode into recombination, the energy of which can allow the fluorescent substance to emit light, thereby providing good visibility. The organic EL element also needs lower power consumption because no backlight source is used, so that it is expected to become the alternative display device, particularly in hand-held device equipment such as cellular phones.
The active matrix organic EL display device needs to increase the light-emitting efficiency of the organic EL element itself by improving the injection efficiency of the holes in the light-emitting layer in the laminated structure such as the hole injection layer/hole-transport layer/light-emitting layer/electron-transport layer, and by improving the generation efficiency of excitons generated in the recombination and the like. In order to improve the display quality in the active matrix organic EL display device, the characteristic of the TFT needs to be improved, such as the circuit response and the like.
In the above-mentioned TFT, amorphous silicon TFT that uses amorphous silicon layer has mainly been used. The requirements for better TFT characteristics, however, have recently led to the development of TFT using the polysilicon layer (hereinafter abbreviated as polysilicon TFT) with higher electric field mobility than the amorphous silicon layer. The manufacture of polysilicon TFT requires a process for the crystallization of the amorphous silicon layer. Such a process includes a high-temperature process for crystallization at a temperature of about 600° C. using an electric heating furnace, and a low-temperature process for crystallization at a low temperature of about 300° C. or less using laser or infrared light.
High-temperature process has the advantages that the LSI (Large Scale Integration) technology can be used to form a thermally-oxidized gate insulating layer and the TFT characteristic variation can be prevented because interface properties between the thermally-oxidized gate insulating layer and the polysilicon are stabilized. The high-temperature process, however, needs higher temperatures during the process of crystallization process, so that it cannot apply to display devices that use a substrate such as glass or plastic. The low-temperature process for crystallization using laser annealing or lamp annealing has thus usually been used for active matrix organic EL display device.
A description is given below of an active matrix organic EL display device with a polysilicon TFT made by the above-described low-temperature process (hereinafter referred to as a low temperature polysilicon TFT). FIG. 1A schematically shows a plan view of the structure of the conventional active matrix organic EL display device described in Japanese application patent laid-open publication No. 2001-318628. FIG. 1B shows a cross sectional view taken along the line B-B′ in FIG. 1A. FIG. 2A to FIG. 2E, FIG. 3A to FIG. 3D, and FIG. 4A to FIG. 4C show process cross sectional views of a series of manufacturing procedures for the active matrix substrate. FIG. 5 shows an equivalent circuit diagram of the active matrix organic EL display device.
Referring first to FIG. 2A to FIG. 4C, a description is given of the manufacturing process of the active matrix substrate described in the above-described reference. First, primary layer 101 is formed on glass substrate 100. Amorphous silicon is then deposited, and polysilicon layer 102 is formed using laser annealing or lamp annealing or the like (see FIG. 2A). Next, on polysilicon layer 102 is formed protective layer 103 of a silicon oxide layer, on which is formed resist mask 104. Using resist mask 104, n-type impurities such as phosphorous or arsenic are added to form n-type impurity region 105. The added impurities are then activated using laser annealing or the like (see FIG. 2B and FIG. 2C). Next, polysilicon layer 102 is locally removed to form island-shaped active layers 106 to 109. Then, after gate insulating layer 110 has been formed, gate electrodes 111 to 114, source wiring 115, and current-supply line 116 are formed (see FIG. 2D and FIG. 2E).
Next, using gate electrodes 111 to 114 as masks, n-type impurities such as phosphorous are doped in a self-alignment manner to form impurity regions 117 to 124. Then, using resist mask 125, n-type impurities such as phosphorous are locally added to form impurity regions 126 to 130 including a high concentration of phosphorous (see FIG. 3A and FIG. 3B). Next, using resist mask 131, p-type impurities such as boron are locally doped to form impurity regions 132 to 135 including a high concentration of boron (see FIG. 3C). Then, resist mask 131 is removed to provide a circuit element such as polysilicon TFTs (see FIG. 3D).
Next, first interlayer insulating layer 136 is formed on the circuit element including polysilicon TFTs, and the impurity element is activated with laser annealing or lamp annealing. Then, second interlayer insulating layer 137 is formed. Contact holes are then formed through first interlayer insulating layer 136, second interlayer insulating layer 137, and gate insulating layer 110, down to the impurity region. Then, metal is filled in each contact hole and patterned to form wirings 138 to 145. Then pixel electrode 146 is formed in contact with connection electrode 141 (see FIG. 4A and FIG. 4B).
The above-mentioned processes in FIG. 2A to FIG. 4B are the same as the low temperature polysilicon TFT manufacturing technology that is used in active matrix liquid crystal display devices and the like. The above-mentioned processes can be achieved by applying technologies such as polysilicon formation technology for forming polysilicon by forming and annealing the amorphous silicon layer, impurity implantation technology for forming n-type TFT and p-type TFT, conductive layer formation technology such as Al and insulating layer formation technology for layers of silicon oxide and silicon nitride, resist layer formation technology for defining these layer-formed regions and implanted regions, and etching technology for removing regions that need no formed layer.
After pixel electrode 146 is formed, third interlayer insulating layer 147 is formed as shown in FIG. 4C. The laminated structure included in the organic EL element except for the anode and cathode has a thickness as thin as about 80 nm to 200 nm, and the cathode has a thickness as thin as about 30 nm to 300 nm, so that steep shapes are covered before forming the organic EL element to prevent cracks from forming at the step edges, and edges are tapered to prevent cracks at the edge steps. After the third interlayer insulating layer 147 is formed in a tapered shape, organic EL layer 148 is formed with vapor deposition technology in the desired region in each pixel, and then, cathode 149 and protective electrode 150 are formed. Finally, passivation layer 151 is formed to protect organic EL layer 148, thereby providing the active matrix organic EL display device.
As shown in the equivalent circuit in FIG. 5, the active matrix organic EL display device formed in the above-described manner has a pixel enclosed by gate wiring 145 disposed in a row direction, and source wiring 115 and current-supply line 116 disposed in a column direction. The pixel contains switching TFT 202 connected to gate wiring 145 and source wiring 115, light-emitting element 204 including organic EL layer 148 sandwiched between pixel electrode (anode) 146 and cathode 149 (see FIG. 4C), and controlling TFT 203 including source and drain; one is connected via holding capacitance 207 to the drain of switching TFT 202, and the other is connected to the anode of light-emitting element 204. Cathode 149 of light-emitting element 204 is common to all pixels. In this way, the cathode electrode may be a single electrode structure across the entire display region, because wiring lines from the row and column drivers can select the address for each pixel, so that cathode 149 is only a power supply electrode.
As shown in FIG. 1A, on substrate 4001, the pixels represented in the above-described equivalent circuit are arranged in a matrix to form pixel portion 4002, and gate-side drive circuit 4004 and source-side drive circuit 4003 are disposed along the ends in a row direction and a column direction. The above components are then sealed with first sealing material 4101 and second sealing material 4104. Wiring 4005 as a lead is formed during the sealing, and one end of FPC (flexible print circuit) 4006 is connected to this wiring 4005. In the organic EL display device thus obtained, as seen from FIG. 1B which shows the cross section taken along the line B-B, in FIG. 1A (a connection with the outside, a part of source-side drive circuit 4003, and a cross section of one pixel of pixel portion 4002), and as seen from FIG. 4C, the cathode electrode (which corresponds to cathode 149 and protective electrode 150 in FIG. 4C, and cathode 4305 in FIG. 1B) is formed across the entire display region where pixels are arranged in a matrix, and thus also formed over pixel circuits including polysilicon TFTs, and over wiring lines connecting the row and column drivers to the pixel circuit.
There are two problems as follows with respect to the structure of the conventional active matrix organic EL display device in which the cathode electrode that constitutes a part of the organic EL elements is formed across the entire display region as a single electrode.
The first problem is that because the cathode electrode is formed across the entire display region and thus also formed over wiring lines connecting the row and column drivers to the pixel circuits, capacitance arises between this wiring line and the cathode electrode, which delays the signals traveling on the wiring line. Such delayed signals may limit the frame frequency, which makes it difficult to provide an active matrix organic EL display device that is adaptable to high-speed moving video pictures. Sending signals to the wiring line that has the above-described capacitance is also disadvantageous in terms of power consumption and may prevent lower power consumption from being achieved.
The second problem is that in the vapor deposition process for forming the cathode electrode, the electron beam vapor deposition source cannot be used as a vapor deposition source. This problem is described more specifically below.
The vapor deposition process is a technology that heats and evaporates materials that are to be coated in a vacuum and deposits them on a substrate. While there are many methods for heating the materials to be coated, the electron beam vapor deposition source is often used for general mass production. This is because, compared to other vapor deposition sources, the electron beam vapor deposition source has a more stable evaporation angle for the materials to be coated which can provide a higher quality vapor deposition layer, and it provides less sprats and more uniform layers, and allows for easier filling of the materials to be coated, and needs less maintenance, which can increase the capacity utilization of the film formation facility.
It has become apparent, however, that when the cathode electrode of the conventional active matrix organic EL display device is formed with the vapor deposition system including the electron beam vapor deposition source, the cathode electrode is formed across the entire display region and thus also formed over the polysilicon TFTs, so that the characteristic X-ray emerging from the electron beam vapor deposition source may degrade the polysilicon TFT characteristics, by changing the threshold voltage Vt, increasing the leak current, decreasing the on-state current and the like.
FIG. 6 shows experimental results of the characteristic change of polysilicon TFTs which are irradiated with X-ray. In FIG. 6, the x-axis shows the gate voltage, and the y-axis shows the drain current. As seen in FIG. 6, X-ray exposure can shift the polysilicon TFT characteristics to the more negative value (to the left in the drawing) for both the pch-TFT and nch-TFT. This gate voltage shift may prevent the normal TFT operation, and make it impossible to realize a high image quality display device without lines or non-uniformity on the screen.
The above-described gate voltage variation of the polysilicon TFTs may arise from the trap levels generated in the gate insulating layer of the polysilicon TFTs. Each of the TFTs arranged in matrix, in conventional polysilicon TFT, particularly the polysilicon TFT made with the low-temperature process, however, lacks sufficient characteristics and uniformity, which makes it impossible to demonstrate the effect of the characteristic X-ray. The inventors of the present invention have demonstrated the effect of the characteristic X-ray by improving the low temperature polysilicon TFT manufacturing technology to make it possible to manufacture the polysilicon TFT with good characteristics and good uniformity thereof. The novel fact that the inventors of the present invention have found is the detailed relationship between the characteristics X-ray from the electron beam vapor deposition and the gate voltage variation of the low temperature polysilicon TFT.
As described above, the conventional active matrix organic EL display device has a cathode electrode that is formed across the entire display region and thus also formed over the wiring lines connecting the row and column drivers to the pixel circuit and over the polysilicon TFTs. This may cause a signal delay problem due to capacitance arising between the wiring lines and cathode electrode, and the polysilicon TFT characteristic degradation problem due to the characteristic X-ray that are generated during the electron beam vapor deposition, thus making it impossible to realize a high-speed-response and high-quality display device.
The present invention was realized in light of the above-described problems, and aims mainly to provide an active matrix organic EL display device and a manufacturing method thereof which can prevent, without complexing the manufacturing process, the display quality decrease caused by the signal delay that occurs due to the capacitance between the wiring lines and cathode electrode and the polysilicon TFT characteristic degradation.